Vibration isolation tables are used to support sensitive instrumentation. In a vibration isolation table a table top is supported by a vibration isolation system. The system comprises air operated vibration isolators, sensors, pneumatic controls and structural supports. These air operated vibration isolators are "air springs" in a general sense in that they utilize the compressibility of air contained in a chamber, a flexible sealing element and a load bearing piston to produce the characteristics of a low frequency spring i.e. a soft suspension for the object supported. Unlike metal springs and shock cords they can be made to accommodate a varying load without major deflection by varying the air pressure (and thus the lift force) in the air chamber.
The isolators work only by supporting an inertial mass. The vibrational forces transmitted through an isolator to the load mass decrease with increased vibrational frequency above the resonance frequency of the air spring support. That is, the transmissibility of the air-spring-and-mass system decreases with the increasing frequency of input above resonance.
To monitor and control the position of the supported mass sensing systems are integrated with the supported load and the isolators. Air control valves typically form part of the sensing systems and are designed to possess the required degree of sensitivity for effective operation. The sensing systems can provide positioning within ten thousands of an inch.
After the inertial mass or supported table top has come to equilibrium in a vibration isolation system, the positioning of the table top is controlled by the sensing system. More particularly in prior art systems a mechanical linkage is provided between an arm which contacts the supported load and a valve which controls the flow of air into and out of the isolator. The arm is biased upwardly by the pressure in the isolator. When a load is placed on the table top the arm deflects downwardly. The linkage tracks the movement of the arm and directly controls the flow of air through the valve in response to the movement of the linkage. When the valve opens, air is introduced into the isolator. The piston supporting the load rises, the arm tracking the movement of the table moves and the linkage follows the arm and ultimately causes the valve to close. Briefly, there is mechanical correspondence between the arm, the linkage and the opening and closing of the valve. In order to provide a stable configuration at the equilibrium point, the control system has a range in the vicinity of the equilibrium point in which air neither enters nor exits the isolator. The range of this dead band determines the ultimate precision with which the isolated mass or supported table can be located.
Generally a sensitivity of about ten thousands of an inch is the best that can be expected. This ten thousands of an inch range is commonly referred to as the dead band, the range the load will move through before a response can be expected. More typically the dead band in prior art systems is 40 to 60 thousands of an inch. To summarize, prior art systems employ a three state control system. These three states correspond to air entering the isolator, air exiting the isolator or a third state in which the isolator is effectively sealed off. The finite extent of this third state leads to the existence of the dead band.
The present invention is directed to a sensing system for monitoring and controlling the position of a pneumatically supported load. The sensor of the sensing system need not engage the supported load. That is, there need be no mechanical correspondence between the supported load and the sensor. A signal from the sensor is conditioned and activates a valve which controls the flow of air into and out of an isolator. The dead band inherent in the state of the art systems is essentially eliminated. In a preferred embodiment, a sensitivity of one micron (4/100 thousands of an inch) is achieved. The sensor is spaced apart from a reference plane or the like and movement between the reference plane and the sensor results in an output signal from the sensor corresponding to the relative movement. In the preferred embodiment the sensor is spaced apart from the pneumatically supported load. In an alternative embodiment the sensor is secured to the pneumatically supported load and spaced apart from a reference line.
Broadly, the invention comprises a sensor responsive to the position of a pneumatically positioned load, the sensor spaced apart from the load and providing an output corresponding to the position or orientation of the load. The output is modified to provide a control signal. A valve is responsive to the modified signal to control the flow of the air into and out of an air isolator which supports the load. The control of the flow of air is such that there is no dead band.
In an alternative embodiment of the invention, the sensor is secured to the supported load and spaced apart from a fixed reference point, plane or line.